The invention relates to a color thermal printer.
FIG. 13 is a diagram showing a main portion of a construction of a conventional color thermal printer disclosed in, e.g., Japanese Patent Unexamined Publication No. Sho. 64-5879; and FIG. 14 is a diagram showing an ink sheet. In FIGS. 13 and 14, reference numeral 1 designates a platen; 2, a thermal head mounted on a radiating plate 2a and having a linearly formed heating body; 3, a print sheet; and 4, an ink sheet. The ink sheet 4 is prepared by coating or printing three (3) colorants, namely, yellow (Y) 4b, magenta (M) 4c, cyan (C) 4d, on a base film 4a in succession so as to correspond respectively to a print area. An ink sheet having four (4) colorants including black (BK) 4e is also available. A head color mark 4f for allowing the head color of each set to be sensed and an identification mark (ID mark) 4g for aligning each color with a print start position and for identifying the type of ink sheet are arranged. The head color mark 4 f and the ID mark 4g are sensor marks. The head color mark 4f and the ID mark 4g, which are sensor marks, are coated with a highly light-shielding paint. Reference numeral 5 designates an ink sheet feed roll; 6, an ink sheet take-up roll; and 7, a sensor for sensing light-shielding at the sensor marks.
An operation of the printer will be described next. Almost simultaneously with the print sheet 3 having been forwarded to the print start position, the ink sheet 4 is also forwarded toward the ink sheet take-up roll 6 from the ink sheet feed roll 5. At this instance, the head color mark 4f is sensed by the sensor 7 and the forwarding of the ink sheet 4 is thus stopped. The platen 1 is rotated at a predetermined speed by applying an electric signal corresponding to an image to the thermal head 2 with the print sheet 3 and the ink sheet 4 interposed between the platen 1 and the thermal head 2, and under this condition, the colorant, yellow 4b, is thermally transferred onto the print sheet 3 first. Then, the thermal head 2 is moved away from the platen 1 and the print sheet 3 is returned to the print start position, and at the same time, the ink sheet 4 is forwarded to the next color. The forwarding of the ink sheet 4 is stopped when the ID mark 4g has been sensed by the sensor 7 in the course of forwarding, and the thermal head 2 is caused to abut against the platen 1 to start printing the next colorant (magenta 4c) on the print sheet 3. The colorants, cyan 4d and black 4e, are similarly thermally transferred to complete the printing.
The conventional color thermal printer and the ink sheet are constructed as described above. The sensor 7 is disposed within a print width of the thermal head 2 since the head color marks 4f and the ID marks 4g are arranged at the boundaries between the colorants. While it is desirable to make the distance between the heat line of the thermal head 2 and the sensor 7 as short as possible, a distance L is by all means required since the radiating plate 2a and a guide roller (no reference numeral is designated) are in the way. As a result, the distance L from the head color mark 4f or the ID mark 4g sensed by the sensor 7 to a colorant to be used for printing becomes a wasteful portion on the ink sheet 4, thus making the outer diameters of the ink sheet feed roll 5 and the ink sheet take-up roll 6 large.
FIG. 15 is a diagram showing the relationship between the ink sheet and the sensor in another conventional example disclosed in, e.g., Japanese Patent Unexamined Publication No. Sho. 64-5879. In FIG. 15, reference characters 4 and 4a to 4g designate the same parts and components as those described above. Since the head color mark 4f and the ID mark 4g are arranged out of the print width of the thermal head 2 in the second conventional example, the print start position can be close to the boundary of each colorant by interposing only a distance L between the sensor 7 and the boundary of each colorant, thus producing no such waste as the distance L in the first conventional example. However, since the sensor 7 is disposed outside the print width of the thermal head 2, the ink sheet 4 must include such margins as to allow the head color marks 4f and the ID marks 4g to be therein arranged in addition to a width W1 of the colorant, which makes the width of the ink sheet 4 as wide as W2 (W1&lt;W2).
Although it depends on how the head color marks 4f and the ID marks 4g are arranged on the ink sheet 4 and on how the sensor 7 is disposed in the printer, the thus constructed conventional color thermal printers have to accommodate either an ink sheet roll whose outer diameter is large or an ink sheet roll whose width in the axial direction is large, which has been a hindrance to downsizing color thermal printers.